1. Field of the Invention
The present invention relates to a method for producing a solid-phase dried or solid-phase polymerized polyamide. More specifically, the present invention relates to a method for producing the solid-phase dried or solid-phase polymerized polyamide with little increase in yellowness by solid-phase drying or solid-phase polymerizing a polyamide comprising a diamine component containing 70 mol % or more of xylylene diamine and a dicarboxylic acid component containing 70 mol % or more of adipic acid.
2. Description of the Prior Art
Polyamides are materials excellent in heat resistance, chemical resistance, mechanical properties and moldability, and have been widely used as synthetic fibers, molding materials for automotive parts, etc., packaging materials such as film, etc. Recently, in various fields of polyamide applications, the development of new products is forwarded to achieve still higher performance, and it is demanded to develop a polyamide having still higher heat resistance, mechanical strength and gas barrier properties as well as showing no reduction in mechanical properties due to water absorption.
As a polyamide having a low water absorption, a high mechanical strength and high gas barrier properties, is known nylon MXD6 that is prepared by the polycondensation of m-xylylene diamine and adipic acid. Japanese Patent Application Laid-Open No. 6-192416 proposes, as a polyamide that is excellent in moldability and mechanical properties even when absorbing water, a polyamide prepared by the polycondensation of p-xylylene diamine, hexamethylene diamine and adipic acid. These polyamides prepared by using xylylene diamine draw attention as a polyamide having a high heat resistance, a high mechanical strength, high gas barrier properties and showing no reduction in mechanical properties due to water absorption.
Since a polyamide is a water-absorptive thermoplastic resin and a solid polyamide absorbs water or moisture in an amount exceeding its saturation amount at molten state, the polyamide should be dried prior to molding or forming process. Alternatively, a dried polyamide is packed into a moistureproof packaging bag for shipping, and then, used as a material for molding or forming. Polyamide is generally dried in a solid state after producing a melt-polymerized polyamide. A melt drying method using an extruder, etc. is disadvantageous in view of the productivity and the coloring or yellowing due to heat history.
As a material for producing solid article, a low viscosity polyamide is used, because the molding is carried out by injection molding that requires polyamide to be highly flowable in a molten state. In contrast, as a material for producing bottle, sheet, film, fiber, etc., a low-, medium- or high-viscosity polyamide is used because these products are produced by extrusion in addition to injection molding, and therefore, a molten polyamide is required to have a flowability lower than used as a materiel for producing solid article. To produce a medium-to-high viscosity polyamide by melt polymerization, the polymerization under molten state should be carried out in a longer period of time, this likely to cause degradation of polyamide due to increased heat history resulted by the prolonged reaction time. In addition, a polymerization apparatus with specific design is needed, because a sufficient power for maintaining the molten state of polyamide in a polymerization tank uniform cannot be obtained by an agitation apparatus for general use. Further, the operation for taking out a molten medium-to-high viscosity polyamide from a polymerization tank is rather difficult as compared with taking out a low-density polyamide, and the amount of a residual polyamide adhering to the inner wall of the polymerization tank becomes larger as compared with the production of a low-density polyamide. Thus, the production of the medium-to-high viscosity polyamide by melt polymerization is undesirable.
Therefore, the medium-to-high polyamide has been generally produced by a so-called solid-phase polymerization where a low-density polyamide is first produced by a melt polymerization and then the low-density polyamide is heat-treated in solid state. Since the degree of polymerization can be increased at temperatures lower than the melting point, the solid-phase polymerization is superior to the melt polymerization in avoiding the coloring due to heat degradation. Therefore, the solid-phase polymerization has been widely used in various manners.
Since polyamide is, as its inherent feature, liable to be colored yellow by oxidative degradation or heat degradation during the molding or forming process, a polyamide as little colored as possible is desired as a material for molding or forming. As a method for preventing the coloring by the oxidative degradation during the solid-phase drying and solid-phase polymerization, there have been known a method where polyamide is treated by heating from a heat transfer surface and a method where polyamide is treated in an inert gas atmosphere such as heated dry nitrogen. Polyamide for use as a starting material is generally subjected to the solid-phase drying and solid-phase polymerization relatively immediately (without storing for a long term) after produced by the melt polymerization, etc. In such a case, it is sufficient for preventing the coloring to take care only to remove oxygen during the solid-phase drying and solid-phase polymerization. However, the oxidative degradation cannot be effectively prevented only by the oxygen removal when a long term storage of a polyamide for use as a starting material is unavoidable, for example, in case of an apparatus for the solid-phase drying or polymerization being troubled thereby disenabling the solid-phase drying or polymerization immediately after the production of the starting polyamide; in case of the solid-phase drying being repeated because of moisture absorption after the solid-phase drying or polymerization; in case of the solid-phase polymerization being repeated to further increase the molecular weight after the solid-phase drying or polymerization; or in case of the melt polymerization for producing the starting polyamide being carried out at a site being far away from the site of the solid-phase drying or polymerization.
Japanese Patent Application Laid-Open No. 2000-129119 discloses a method for spraying water onto the surface of polyamide that is produced by a melt polymerization. This method is intended to prevent the polyamide from contacting air by covering the surface of polyamide with surplus water after increasing the water content of polyamide. However, to obtain the effect continuously, a water film must be maintained on polyamide by continuously supplying excess water over the saturated amount, this being not practical for a long term storage. In addition, such an excess water necessitates a significantly large heat load and a prolonged time in the subsequent solid-phase drying or polymerization step. Thus, the proposed method is industrially disadvantageous.
It has been known to add an polyamide with a stabilizing agent such as phosphoric acid, phosphorous acid, hypophosphorous acid, and salts and compounds thereof to prevent the heat degradation or the oxidative degradation (Japanese Patent Publication No. 48-23199). However, the coloring during the solid-phase drying or solid-phase polymerization cannot be prevented sufficiently even by the addition of such stabilizing agents. In addition, since the stabilizing agent acts also as a polymerization promoter for polyamide, the addition of the stabilizing agent in an amount larger than needed is undesired in view of controlling the degree of polymerization.
An object of the present invention is to provide a method for solid-phase drying or solid-phase polymerizing polyamide with a low increase in the yellowness.
As a result of intensive study in view of solving the above problem, the inventors have found that the yellowing of a polyamide due to the oxidative degradation during a solid-phase drying or a solid-phase polymerization, even after stored for 20 days or longer from the production of the polyamide, can be effectively prevented by controlling the oxygen partial pressure, the relative humidity and the temperature of the atmosphere to which the polyamide being stored is exposed, the term for storage and the crystallization degree of the polyamide being stored, or by pre-treating the polyamide under specific conditions with a decreased oxygen partial pressure prior to the solid-phase drying or the solid-phase polymerization. The present invention has been accomplished on the basis of this finding.
Thus, the present invention provides a method for solid-phase drying or solid-phase polymerizing a polyamide that comprises a diamine component containing 70 mol % or more of xylylene diamine and a dicarboxylic acid component containing 70 mol % or more of adipic acid, the method comprising solid-phase drying or solid-phase polymerizing the polyamide that is stored for 20 days or longer after the production thereof until subjected to the solid-phase drying or the solid-phase polymerization under conditions satisfying the following Formula A:
0.8xc3x9710xe2x88x929xe2x89xa7Pxc3x97D0.5xc3x97exp{xe2x88x926002/T+(1xe2x88x922xc3x97C/100)xc3x97(axc3x97(h/100xe2x88x920.6)2+0.62)}xe2x80x83xe2x80x83(A) 
wherein P is an oxygen partial pressure (MPa) of an atmosphere to which the polyamide is exposed; h is relative humidity (%RH) of the atmosphere to which the polyamide is exposed; D is a storage time (days); T is a temperature (K) of the atmosphere to which the polyamide is exposed; C is a crystallization degree (%) of the polyamide; a is a numerical value of 2.9 when h is less than 60%RH or 18.8 when h is 60%RH or higher, at a temperature lower than a glass transition temperature of the polyamide while shielding against light.
The present invention further provides a method for solid-phase drying or solid-phase polymerizing a polyamide that comprises a diamine component containing 70 mol % or more of xylylene diamine and a dicarboxylic acid component containing 70 mol % or more of adipic acid, the method including a step of exposing the polyamide to an atmosphere having an oxygen partial pressure of 1 kPa or less at 120xc2x0 C. or lower for 2 h or more in an apparatus.